Technical Field
This technology relates to closed woven composite articles formed by thermally fusing an open woven fabric formed from high tenacity, thermoplastic elongate bodies that are loosely interwoven with binding fibers, and to a continuous process for forming the composite articles.
Description of the Related Art
High tenacity fibers, such as SPECTRA® polyethylene fibers or aramid fibers such as KEVLAR® and TWARON® fibers, are known to be useful for the formation of articles having excellent ballistic resistance. Ballistic resistant articles formed from high tenacity tapes are also known. Articles such as bullet resistant vests, helmets, vehicle panels and structural members of military equipment are typically made from fabrics comprising high tenacity fibers or tapes because of their very high strength to weight performance. For many applications, the fibers or tapes may be formed into woven or knitted fabrics. For other applications, the fibers or tapes may be encapsulated or embedded in a polymeric matrix material and formed into non-woven fabrics. In one common non-woven fabric structure, a plurality of unidirectionally oriented fibers are arranged in a generally coplanar, coextensive relationship and coated with a binding matrix resin to bind the fibers together. Typically, multiple plies of such unidirectionally oriented fibers are merged into a multi-ply composite. See, for example, U.S. Pat. Nos. 4,403,012; 4,457,985; 4,613,535; 4,623,574; 4,650,710; 4,737,402; 4,748,064; 5,552,208; 5,587,230; 6,642,159; 6,841,492; and 6,846,758, all of which are incorporated herein by reference to the extent consistent herewith.
Composites fabricated from non-woven fabrics are known to stop projectiles better than woven fabric composites because the component fibers in non-woven fabrics are not crimped like the fibers in woven materials. Fiber crimping reduces the ability of the fibers to stay in tension and immediately absorb the energy of a projectile, compromising their effectiveness. In addition, projectile damage to non-woven fabrics is more localized compared to woven fabrics, allowing for enhanced multi-hit performance. However, non-woven composite technology remains imperfect. Traditional non-woven composites are not ideal because the resin coating that is generally necessary to keep the component fibers bound together is present in place of a greater quantity of high tenacity fibers. The reduction in overall fiber content reduces the maximum achievable ballistic resistance efficiency on an equal weight basis relative to fabrics incorporating no resin coating. However, it is difficult to produce single-ply sheets of unidirectionally oriented fibers with adequate mechanical integrity when less than 10% by weight of bonding resin is used.
In addition, to maximize ballistic resistance, it is desired for there to be a bare minimum of space between adjacent fibers to facilitate maximum engagement of the fibers with a projectile threat. One way to accomplish that is by adding more fibers to a fibrous layer, but that makes the armor heaver, which is undesirable. A more preferred method is spreading filaments apart to form thinner fiber layers having fewer fibers that lie on top of each other. This allows a greater number of fiber layers to be stacked on top of each other without altering the expected fabric thickness, thereby enhancing fiber engagement with projectile threats without increasing fabric weight. However, it is difficult to produce single-ply sheets of unidirectionally oriented fibers with adequate mechanical integrity when the filaments of the fibers are spread very thinly.
One method of addressing this problem of inadequate mechanical integrity during composite fabrication is to use a release paper carrier sheet during processing. In a typical process, an array of unidirectionally oriented parallel fibers is coated with a binder resin and then the coated fibers are contacted with a silicone-coated release paper while the resin is still wet. The coating is then dried and the release paper is removed. However, this method also has associated disadvantages and it is desired to avoid the use of a carrier sheet in the manufacturing process. Accordingly, there is an ongoing need in the art for an improved ballistic resistant composite that combines the superior mechanical strength of woven fabrics with the superior ballistic resistance of non-woven fabrics.
In this regard, U.S. Pat. No. 8,349,112 teaches a method of weaving polymeric tapes together with binding threads, with the polymeric tapes being used as warp yarn and a binding thread being used as weft yarn or with the polymeric tapes being used as weft yarn and a binding thread being used as warp yarn, followed by consolidating multiple layers with sufficient heat to melt the binding threads. The melting deforms the binding threads, distributing the resin around the non-melted polymeric tapes, thereby acting as an adhesive coating. This eliminates the undulations caused by the weaving process. However, this method does not produce articles having less than 10% resin content with sufficient mechanical integrity. U.S. Pat. No. 8,349,112 is silent with regard to binding resin content, but the thermal destruction of the binder fibers compromises the fabric breaking strength in the direction transverse to the polymeric tapes. The melting of the binder fibers eliminates the mechanical interlocking of warp and weft fibers created by the weaving process, resulting in a non-woven fabric with the binder polymer serving as a conventional adhesive coating. This resulting fabric either has greater than 10% resin content or less than 10% resin content and inadequate mechanical integrity, thereby failing to improve upon prior art composites. Accordingly, U.S. Pat. No. 8,349,112 fails to achieve the objectives of the present invention.
U.S. Pat. No. 4,680,213 teaches structures where non-thermoplastic, reinforcing textile yarns are bonded by adhesion with binding yarns disposed transverse to the textile yarns. The reinforcing textile yarns are spaced apart from each other and the binding yarns are spaced apart from each other, so as to form permanent holes in their laminates. This type of open structure is unacceptable for anti-ballistic applications, and is not described as having utility as a ballistic resistant composite.
Accordingly, there is an ongoing need in the art for a ballistic resistant composite containing less than 10% binder resin and having reduced thickness that combines the superior mechanical strength of woven fabrics with the superior ballistic resistance of non-woven fabrics. The present invention provides a solution to this need.